Switch mode energy recovery for electro-luminescent lamp panels

ABSTRACT

High voltage and low voltage switch mode circuits and methods serve to recover the charge stored on electro-luminescent lamp panels that would otherwise be dissipated during the discharge cycle of a drive circuit. The high voltage circuits and methods operate to transfer the charge to the high voltage rail, while the low voltage circuits and methods operate to transfer the charge to the source of low drive voltage.

FIELD OF THE INVENTION

[0001] This invention relates generally to electro-luminescent lamppanels and, more particularly, to the recovery of energy stored in thesepanels that is otherwise dissipated during the discharge cycle of adrive circuit.

BACKGROUND AND SUMMARY OF THE INVENTION

[0002] Electro-luminescent lamps act as capacitors, electrically. Theselamps store energy, as do all capacitors, in the form of an electricalvoltage charge. In the normal electro-luminescent lamp driver circuit,this charge is dissipated, and therefore lost, during the dischargecycle of operation.

[0003] Electro-luminescent lamp driver circuits are well known in theprior art, exemplary of which are the Supertex HV803 and the TokoTK659XX. A typical one of these circuits is illustrated in FIG. 1. Inthat circuit, components L1, S0, D1, and C1 constitute a high voltageboost or step-up converter which receives a low voltage (less than 6volts) and boosts it to between 20 and 100 voltas on the capacitor C1.Components S1, S2, S3, and S4 constitute a an H-bridge circuit that isused to commutate the high DC voltage on capacitor C1 into a high ACvoltage across an electro-luminescent lamp capacitor (C lamp) that isabout twice the DC voltage on capacitor C1. This AC voltage charges anddischarges the capacitor C lamp, with the energy stored in the capacitorC lamp being dissipated in components S3 and S4 of the H-bridge duringthe discharge cycle.

[0004] It would be advantageious to recover this energy, existing in theform of charge at high voltage, from the electro-luminescent lampcapactior C_lamp, and reuse it, thus making the entireelectro-luminescent lamp driving system more efficient. Accordingly, thepresent invention is directed to circuitry required to implement therecovery of this otherwise lost energy, which is significant in the caseof large electro-luminescent lamps that are driven to higher voltages.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005]FIG. 1 is a circuit diagram of a typical prior artelectroluminescent lamp driver circuit that is unable to recover chargefrom the electro-luminescent lamp capacitor.

[0006]FIG. 2 is a schematic diagram of an electro-luminescent lampdriver circuit employing a high voltage switch mode method of chargerecovery, in accordance with one embodiment of the present invention.

[0007]FIG. 3 is a schematic diagram of an electro-luminescent lampdriver circuit employing an energy transfer capacitor, in accordancewith another embodiment of the present invention.

[0008]FIG. 4 is a schematic diagram of an electro-luminescent lampdriver circuit employing an integrated high voltage switch mode methodof charge recovery, in accordance with another embodiment of the presentinvention.

[0009]FIG. 5 is a waveform diagram illustrating typical waveformsgenerated by the electro-luminescent lamp driver circuits of FIGS. 2-4.

[0010]FIG. 6 is a schematic diagram of an electro-luminescent lampdriver circuit employing charge pump energy recovery, in accordance withanother embodiment of the present invention.

[0011]FIG. 7 is a waveform diagram illustrating simulated waveformresults obtained from the electro-luminescent lamp driver circuit ofFIG. 6.

[0012]FIG. 8 is a schematic diagram of an electro-luminescent lampdriver circuit employing a low voltage switch mode method of energyrecovery, in accordance with another embodiment of the presentinvention.

[0013]FIG. 9 is a schematic diagram illustrating the circuit equivalentof the circuit of FIG. 8 during a first cycle of operation.

[0014]FIG. 10 is a schematic diagram illustrating the circuit equivalentof the circuit of FIG. 8 during second and fourth cycles of operation.

[0015]FIG. 11 is a schematic diagram illustrating the circuit equivalentof the circuit of FIG. 8 during a third cycle of operation.

[0016]FIG. 12 is a waveform diagram illustrating the simulation ofelectroluminescent lamp panel energy being returned to the low voltagesource as a current, with the frequency of the transfer controller clockbeing constant with time.

[0017]FIG. 13 is a waveform diagram illustrating the simulation ofelectroluminescent lamp panel energy being returned to the low voltagesource as a current, with the frequency of the transfer controller clockincreasing with time.

[0018]FIG. 14 is a schematic diagram of an electro-luminescent lampdriver circuit employing a low voltage switch mode method of energyrecovery, integrated with a boost converter, in accordance with anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0019] Described below are a number of high and low voltage methods forthe recovery of otherwise dissipated energy from electro-luminescentlamp panels. The high voltage methods generally return energy to thehigh voltage node of the electro-luminescent lamp driver circuit or tothe electro-luminescent capacitor, while the low voltage methods returnenergy to a low voltage source.

[0020] Referring now to FIG. 2, there is shown an electro-luminescentlamp driver circuit employing a high voltage switch mode method ofcharge recovery in which a second boost converter is employed todischarge the voltage on the electro-luminescent lamp and return energyto a capacitor C1. This method increases the peak-to-peak voltage to theelectro-luminescent lamp. An H-bridge operates normally to charge acapacitor C_lamp, as shown in the waveform diagram of FIG. 5. Indischarging capacitor C_lamp, only switch S1 is opened, while switch S5is closed, thereby applying the voltage on capacitor C_lamp to the inputof the boost converter that employs components L2, S7, and D2. Theswitching frequency and duty cycle of switch S7 is selected to effectthe most efficient transfer of energy from capacitor C_lamp to capacitorC1. After most of the charge on capacitor C_lamp is transferred, switchS5 opens and switch S3 closes, thereby discharging capacitor C_lampcompletely. The charging cycle is repeated on the other side of theH-bridge by opening switch S4 and closing switch S6, while operatingswitch S7 as described above. After capacitor C-lamp is nearlydischarged, switch S6 opens and switch S4 closes, following which switchS3 opens and switch S1 closes, thus beginning another AC voltage cycleof capacitor C_lamp. The advantage of charge recovery is realized as anincreased rate of rise in the voltage on capacitor C1, as shown in thewaveform diagram of FIG. 5, when switches S5 or S6 are closed andcapacitor C_lamp is being discharged.

[0021] Referring now to FIG. 3, there is shown an electro-luminescentlamp driver circuit employing an alternative high voltage switch modemethod of charge recovery in which an energy transfer capacitor C_ET isprovided to the second boost converter to control the rate of dischargeof the electroluminescent lamp panel voltage. Switches S8-S11 arecontrolled in a way similar to that described below in connection withFIGS. 9-11.

[0022] Referring now to FIG. 4, there is shown an electro-luminescentlamp driver circuit integrated with the main boost converter as analternative high voltage switch mode method of charge recovery. Thiscircuit realizes a saving in the number of external components, butrequires synchronization of timing between the charge recovery circuitand the main boost converter. Inductors L1 and L3 may utilize the samemagnetic core. Operation of the circuit of FIG. 4 is similar to that ofthe electro-luminescent lamp driver circuit of FIG. 2 with regard toswitches S0-S6. However, there is a difference with regard to operationof switch S8, in that it must be synchronized with switch S0. Switch S8must close after switch S0 closes, in order that the current beginningto flow through inductor L3 flows through switch S0 rather than backthrough inductor L1 to the low voltage supply Vdd. In addition, switchS8 must remain closed for a short time after switch S0 open to insurethat the voltage at the anode of diode D1 has decayed to a level belowlow voltage supply Vdd. Switch S8 should then open.

[0023] Referring now to FIG. 6, there is shown an electro-luminescentlamp driver circuit employing a holding capacitor C_ER to which energyis transferred in a particular cycle and from which energy is returnedto capacitor C_lamp in a later cycle. Considering an initial state ofthis circuit in which switches S2 and S3 are closed, while all otherswitches are open, capacitor C_lamp is charged to a voltage at highvoltage rail HV_node by a current flowing through diode D2 and switchesS2 and S3. Next, switches S2 and S3 open, and switch S5 closes, therebytying the right terminal of capacitor C_lamp essentially to the highvoltage rail HV_node and moving the voltage at the left terminal ofcapacitor C_lamp above the voltage at high voltage rail HV_node by someamount. During this phase, diode D4 becomes forward biased, and acurrent flows from capacitor C_lamp into energy recovery capacitor C_ER.The voltage on capacitor C_ER continues to rise until an equilibriumvoltage is reached. Switch S4 now closes, while switch S5 remainsclosed. Capacitor C_lamp is then charged to the voltage at the highvoltage rail HV_node by a current flowing through diode D3 and switchesS4 and S5. Next, a portion of the energy in capacitor C_ER is returnedto capacitor C_lamp by closing switch S7. Operation of the circuit ofFIG. 6, as described above, is illustrated in the simulated waveformresults diagram of FIG. 7.

[0024] Referring now to FIG. 8, there is shown an electro-luminescentlamp driver circuit employing a low voltage switch mode method of chargerecovery in which a current is returned to the low voltage supply V bytransferring energy from the electro-luminescent lamp panel through aswitched capacitor/inductor arrangement. This circuit is referred to asa current limited circuit, since the return current is set by thetopology and clock rate of the circuit. A group of four switching cyclesare used repetitively to accomplish a transfer of energy between theelectro-luminescent lamp panel and the low voltage supply V.

[0025] Initially, a charge or voltage exists on the capacitor C_lamp.This charge may have been deposited earlier by an H-bridge circuitsimilar to that described above, or by some other circuit configuration.An energy transfer capacitor C_ET is assumed to have no initial charge.An energy transfer inductor L_ET may or may not have an initial current.

[0026] Referring now to FIG. 9, during the first cycle of operation ofthe circuit of FIG. 8, switches S1 and S2 close to allow current tobegin flowing from capacitor C_lamp through switch S1, capacitor C_ET,switch S2, inductor L_ET, and the low voltage supply V. During thiscycle, energy is being delivered to the low voltage supply V by means ofa current flowing into its positive terminal.

[0027] After some period of time, a transfer controller opens switchesS1 and S2, to begin a second cycle of operation of the circuit of FIG.8, as illustrated in FIG. 10. At this point, a charge has been developedon capacitor C_ET, and a current is flowing in inductor L_ET. Thiscurrent continues to flow in a recirculation diode DR, while energysupplied by inductor L_ET is being delivered to the low voltage supplyV. The charge on capacitor C_ET and energy (0.5 CV²) are relativelyconstant during this second cycle of operation.

[0028] During a third cycle of operation of the circuit of FIG. 8,illustrated in FIG. 11, the transfer controller closes switches S3 andS4 to cause a current flow from capacitor C_ET through switch S3,inductor L_ET, switch S4, and back to capacitor C_ET. The energy storedin capacitor C_ET is now being delivered through inductor L_ET to thelow voltage supply V.

[0029] During a fourth cycle of operation of the circuit of FIG. 8,illustrated in FIG. 10, the transfer controller opens switches S3 andS4. At this point, the voltage on capacitor C_ET is zero. The currentflowing through inductor L_ET continues to flow in recirculation diodeDR, resulting in a transfer of energy from inductor L_ET to the lowvoltage supply V.

[0030] The four cycles described above are repeated to incrementallytransfer energy from capacitor C_lamp to the low voltage supply V. Eachincremental transfer is approximated by the expression E=0.5*C_ET*(Vel)². In some implementations of the circuit of FIG. 8, only the firstand third cycles are performed, back to back.

[0031] The voltage on capacitor C_lamp drops incrementally followingeach group of four cycles by the amount Vint*(C_lamp/(C_lamp+C_ET)). Byregulating how often each group of four cycles is repeated, the transfercontroller can adjust the rate of decay of the voltage on capacitorC_lamp. Capacitor C_ET facilitates this control at practical switchingfrequencies. For example, without capacitor C_ET and using only inductorL_ET, the switch network may need to operate at approximately twentytimes the normal frequency to achieve the same discharge profile ofcapacitor C_lamp.

[0032] Referring now to FIG. 12, there is shown a simulation during a 1msec. time duration of the operation of the circuit of FIG. 8, givencertain parameters. The initial charge on the 80 nf. capacitor C_lamp is100 volts. The transfer controller is a dual clock source, with the twooutputs being 180 degrees out of phase. The clock frequency is constantat 1 mHz. The upper waveform shows the current flow back into the lowvoltage source V. The center waveform shows the exponential voltagedecay on capacitor C_lamp as energy is being removed from it.

[0033] Alternatively, the frequency of the transfer controller can varywith time, thus producing a different waveform of the discharge ofcapacitor C_lamp, as illustrated in FIG. 13. The waveforms of FIG. 13resulted from the same circuit as those of FIG. 12, except that thefrequency of transfer controller increased linearly with time, thusproducing a convex waveform of voltage decay on capacitor C_lamp asenergy is being removed from it.

[0034] In an alternative embodiment of the circuit of FIG. 8, inductorL_ET can serve as a boost converter inductor L_ET_B, as illustrated inFIG. 14. In this arrangement, a first topology involving inductorL_ET_B, in combination with a switch Boost_FT, forms a step upconverter. Alternatively, a second topology involving inductor L_ET_B incombination with capacitor C_ET, switches S1-S4, and a transfercontroller, forms a low voltage energy recovery circuit. For example,consider a system which repetitively uses two operational phases toproduce an AC waveform across capacitor C_lamp. In a first phase, thefirst topology described above is employed, whereby capacitor C_lamp ischarged by the output of the step up converter, possibly through aswitching bridge, as illustrated in FIG. 1. In a second phase, thesecond topology described above is employed, and capacitor C_lamp isdischarged by the energy recovery topology and method illustrated inFIG. 8. These two phases are repeated at a certain rate, causing thevoltage across capacitor C_lamp to rise and fall at a desired frequency.

We claim:
 1. A circuit for driving an electro-luminescent lamp panel,the circuit employing a high voltage switch mode for recovering chargestored on the electro-luminescent lamp panel, during a discharge cycleof operation of the circuit, the circuit comprising: a device coupled toa source of low voltage for driving the electro-luminescent lamp, thedevice comprising a first capacitor and a switching circuit forswitchably connecting the electro-luminescent lamp panel across thefirst capacitor; and a step-up converter, coupled to theelectro-luminescent lamp panel and the first capacitor for transferringcharge from the electro-luminescent lamp panel to the first capacitorduring the discharge cycle of operation.
 2. A circuit as in claim 1,wherein the step-up converter operates synchronously with the device. 3.A circuit as in claim 1, further comprising a second capacitor incombination with a plurality of switches for controlling the rate oftransfer of charge from the electro-luminescent lamp panel to the firstcapacitor during the discharge cycle of operation.
 4. A circuit as inclaim 2, further comprising a second capacitor in combination with aplurality of switches for controlling the rate of transfer of chargefrom the electro-luminescent lamp panel to the first capacitor duringthe discharge cycle of operation.
 5. A circuit for driving anelectro-luminescent lamp panel, the circuit employing a high voltageswitch mode for recovering charge stored on the electro-luminescent lamppanel, the circuit comprising: a device coupled to a source of lowvoltage for driving the electro-luminescent lamp, the device comprisinga first capacitor and a switching circuit coupled to theelectro-luminescent lamp panel; and a second capacitor, switchablycoupled to the electro-luminescent lamp panel, to which charge istransferred from the electro-luminescent lamp panel during a particularcycle of operation of the circuit, and from which charge is transferredto the electro-luminescent lamp panel during a later cycle of operationof the circuit.
 6. A circuit for driving an electro-luminescent lamppanel, the circuit employing a low voltage switch mode for recoveringcharge stored on the electro-luminescent lamp panel, during one or morecycles of operation of the circuit, the circuit comprising: a source oflow voltage; an energy transfer inductor, coupled to the source of lowvoltage; and a transfer controller, switchably coupled to the energystorage inductor and the electro-luminescent lamp panel, forcontrollably transferring charge from the electro-luminescent lamp panelto the source of low voltage.
 7. A circuit as in claim 6, furthercomprising an energy transfer capacitor, switchably coupled to theenergy transfer inductor and to the electoluminescent lamp panel.
 8. Acircuit for driving an electro-luminescent lamp panel, the circuitemploying a low voltage switch mode for recovering charge stored on theelectroluminescent lamp panel, during one or more cycles of operation ofthe circuit, the circuit comprising: a source of low voltage; an energytransfer capacitor, switchably coupled to the source of low voltage andto the electo-luminescent lamp panel; and a transfer controller,switchably coupled to the energy storage capacitor and theelectro-luminescent lamp panel, for controlling the transfer of chargefrom the electro-luminescent lamp panel to the source of low voltage. 9.A circuit as in claim 6, wherein the transfer controller operates at aconstant frequency.
 10. A circuit as in claim 9, wherein the transfercontroller comprises a dual clock source having two outputs 180 degreesout of phase with each other.
 11. A circuit as in claim 7, wherein thetransfer controller operates at a constant frequency.
 12. A circuit asin claim 11, wherein the transfer controller comprises a dual clocksource having two outputs 180 degrees out of phase with each other. 13.A circuit as in claim 8, wherein the transfer controller operates at aconstant frequency.
 14. A circuit as in claim 13, wherein the transfercontroller comprises a dual clock source having two outputs 180 degreesout of phase with each other.
 15. A circuit as in claim 6, wherein thetransfer controller operates at a frequency that varies with time.
 16. Acircuit as in claim 7, wherein the transfer controller operates at afrequency that varies with time.
 17. A circuit as in claim 8, whereinthe transfer controller operates at a frequency that varies with time.18. A circuit as in claim 15, wherein said frequency increases linearlywith time.
 19. A circuit as in claim 16, wherein said frequencyincreases linearly with time.
 20. A circuit as in claim 17, wherein saidfrequency increases linearly with time.
 21. A circuit for driving anelectro-luminescent lamp panel, the circuit employing a low voltageswitch mode for recovering charge stored on the electro-luminescent lamppanel, during one or more cycles of operation of the circuit, thecircuit comprising: a source of low voltage; a step-up converter coupledto a source of low voltage for driving the electro-luminescent lamp, thestep-up converter comprising an inductor, a switch, a diode, and a firstcapacitor; a second capacitor, switchably coupled to the inductor; and atransfer controller, switchably coupled to the inductor, the secondcapacitor, and the electro-luminescent lamp panel, for controlling thetransfer of charge from the electro-luminescent lamp panel to the sourceof low voltage.
 22. A method for recovering energy stored on anelectro-luminescent lamp panel, the method comprising: applying a firstvoltage present across the electro-luminescent lamp panel to an input ofa switch-mode energy conversion circuit; operating the switch-modeenergy conversion circuit to transform energy representative of saidfirst voltage to a second voltage that is higher than said firstvoltage; applying said second voltage to a storage device; anddelivering energy stored in said storage device back to saidelectro-luminescent lamp panel.
 23. A method for recovering energystored on an electro-luminescent lamp panel, the method comprising:applying a first voltage present across the electro-luminescent lamppanel to an input of a switch-mode energy conversion circuit; operatingthe switch-mode energy conversion circuit to transform energyrepresentative of said first voltage to a second voltage that is higherthan said first voltage; applying said second voltage to a storagedevice; and delivering energy stored in said storage device to anelectro-luminescent lamp panel driver circuit.
 24. A method forrecovering energy stored on an electro-luminescent lamp panel, themethod comprising: applying a first voltage present across theelectro-luminescent lamp panel to an input of a switch-mode energyconversion circuit; operating the switch-mode energy conversion circuitto transform energy representative of said first voltage to a secondvoltage that is higher than said first voltage; applying said secondvoltage to a storage device; and delivering energy stored in saidstorage device back to said electroluminescent lamp panel and to anelectro-luminescent lamp panel driver circuit.
 25. A method forrecovering energy stored on an electro-luminescent lamp panel, themethod comprising: applying a first voltage present across theelectro-luminescent lamp panel to an input of a switch-mode energyconversion circuit; operating the switch-mode energy conversion circuitto transform energy representative of said first voltage to a current;delivering said current to a source of operating voltage for saidelectro-luminescent lamp panel to thereby realize a reduction in averageoutput current required from said source of operating voltage to operatesaid electro-luminescent lamp panel.
 26. A method as in claim 25,further comprising the step of controlling an amplitude versus timecharacteristic of said current delivered to said source of operatingvoltage.
 27. A circuit as in claim 1, wherein said device comprises aprimary step-up converter, including an inductor, a switch, and a diode.28. A circuit as in claim 5, wherein said device comprises a primarystep-up converter, including an inductor, a switch, and a diode.